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Strong chiral coupling of adjacent nanomagnets achieved

By Eva Karatairi June 4, 2019
chirally-coupled-nanomagnets_642
Artificial magnetic structures that can be created by exploiting the new coupling mechanism. (a) Synthetic one-dimensional antiferromagnet consisting of a chain of ferromagnetic cobalt dots with alternating out-of-plane orientation (red) connected by alternating in-plane segments (blue). (b) Synthetic skyrmions with different numbers of windings. (c) Checkerboard pattern of laterally coupled nanomagnets forming domains (green and purple shading). (d) Kagome pattern of laterally coupled nanomagnets with frustrated interactions. Credit: Science

Scientists in Switzerland have fabricated layers of nanomagnets with alternating in-plane and out-of-plane magnetization in adjacent regions. This allows for strong coupling between adjacent nanomagnets. Their work provides a platform to design arrays of correlated nanomagnets and to achieve all-electric control of planar logic gates and memory devices for applications.

“On surfaces, the only coupling that is usually present for assemblies of nanofabricated nanomagnets is the dipolar coupling, the dipolar interaction that holds a fridge magnet to a fridge door. But at such small sizes [nanoscale], this interaction is really very weak,” says Laura Heyderman, professor of mesoscopic systems at Paul Scherrer Institute and ETH Zürich, Switzerland and co-author of the study published in a recent issue of Science.

Below a critical size, around 100 nm, the behavior of the nanomagnets—magnetic structures at the nanoscale—is dominated by  the interfacial Dzyaloshinskii-Moriya interaction (DMI). DMI gives the chiral coupling between in-plane (IP) and out-of-plane (OOP) magnetized regions. To understand this, one can imagine that for two adjacent nanomagnetic regions, one with IP and one with OOP magnetization, the magnetic moment will be parallel to the nanomagnet layer in the first case and perpendicular in the second. In between there is a chiral domain wall, where the magnetization transitions from IP to OOP, so rotating from the parallel to the perpendicular orientation. This rotation is set by the DMI to be clockwise or counterclockwise, depending on whether the DMI is negative or positive, which is why it is called a chiral coupling. The sign and strength of the DMI is in turn dependent on the magnetic materials that are used. Zhaochu Luo who is a postdoctoral researcher in Heyderman’s group, collaborating with Pietro Gambardella who is a professor at ETH Zürich and an expert in fundamental magnetism and spintronics, and their colleagues, measured a surprisingly strong chiral interaction, much stronger than the dipolar interaction between nanomagnets of the same size and spacing. 

“With a strong chiral coupling at our fingertips, we were able to fabricate synthetic antiferromagnets of different shapes and dimensions. We can now design, for example, straight or curved chains of antiferromagnetically aligned nanomagnets (so-called synthetic antiferromagnets), square lattices with antiferromagnetic checkerboard patterns, and even synthetic skyrmions [a particular class of chiral magnetization structure],” Heyderman says.

According to Heyderman, the strength of the chiral coupling also means that, if one nanomagnet switches, the next one in line will also switch, and so on. “This enabled us to realize field-free switching of the nanomagnets using electric currents, which is really interesting for spintronic applications,” she says. 

At the beginning, the researchers needed arrays of nanomagnets that would combine strong DMI with regions exhibiting alternating in-plane and out-of-plane magnetic anisotropy. For this, they fabricated arrays of platinum/cobalt/aluminium oxide trilayers (Pt, 6 nm/Co, 1.6 nm/AlOx) patterned by means of electron beam lithography and argon ion milling. 

To ensure that the two regions of cobalt had orthogonal magnetizations and that the coupling between them was strong, the researchers had to find the right balance between DMI and the magnetic anisotropy. They achieved this by carefully controlling the thickness of the cobalt and by making a very sharp, defect-free boundary between the in-plane and out-of-plane magnetized regions. 

“Another challenge was to directly observe the chiral coupling and determine its chirality,” says Heyderman, adding that they looked for a high spatial resolution imaging method that could help them tell whether the magnetization was pointing in-plane or out-of-plane from the contrast in the images. The researchers were able to map out the three-dimensional (3D) magnetization orientation in the nanomagnets by means of x-ray photoemission electron microscopy, using synchrotron x-rays at the SIM beamline of the Swiss Light Source.

“The great control that this precise lateral patterning process provides opens an exciting way to realize new types of complex chiral nanomagnetic systems, not currently accessible via other routes,” says Amalio Fernández-Pacheco, researcher at the University of Glasgow, UK, who was not involved in the study. “The high efficiency provided by current-induced spin-orbit torques to reverse the magnetic state of in-plane/out-of-plane compound areas, and the realization of topologically-complex artificial spin systems, is particularly appealing,” he says.

Christopher Marrows, professor of condensed-matter physics at the University of Leeds, UK says, “This work takes the well-known idea of chiral 180° domain walls in perpendicularly magnetized ultrathin magnetic layers and uses anisotropy engineering to cut one in half and create chiral 90° domain walls fixed at the point where the anisotropy changes direction. This creates chiral nanomagnetic objects that can be configured in a rich variety of 1D and 2D patterns, leading to an equally rich variety of physical phenomena spanning [areas] such as field-free switching for magnetic memory technologies to artificial spin ices for new discoveries in statistical mechanics.” 

Heyderman expects that their research will stimulate further investigations into the DMI and how to tune it not only in transition metal thin films, but also in 2D heterostructures or bulk materials. Their concept for chiral coupling also provides a platform to design novel coupled nanomagnetic systems in planar geometries. “This concept opens completely new opportunities to fabricate all-electric nanomagnet logic devices and reconfigurable magnonic crystals (the magnetic equivalent to photonic crystals), as well as to create model systems to study frustration in artificial spin ices and topological structures such as skyrmions. We are doing first tests in these directions as we speak,” Heyderman says. 

Read the abstract in Science.